1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the same, which are suitable for a television camera, a video camera, a digital still camera, and a silver-halide film camera, for example.
2. Description of the Related Art
A zoom lens having a large aperture ratio, a high zoom ratio, and high optical performance is required for an image pickup apparatus such as a television camera, a silver-halide film camera, a digital camera, or a video camera.
Of those cameras, a zoom lens for the television camera is relatively easy to increase the angle of field and magnification and to reduce the size and weight, and hence there has conventionally often been used a four-unit zoom lens therefor. The four-unit zoom lens includes, in order from an object side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power which moves for zooming, a third lens unit for correcting an image plane that varies with the zooming, and a fourth lens unit having the positive refractive power for imaging.
In recent years, there has been proposed a zoom lens for the television camera in which the fourth lens unit having a small weight and volume constitutes a movable unit for focusing. In this manner, the size of the movable mechanism for focusing may be reduced, and the weight of the zoom lens may be suppressed.
For example, Japanese Patent Application Laid-Open Nos. H04-078807 and 2002-169091 each propose a zoom lens including a first lens unit having a positive refractive power, a second lens unit having a negative refractive power which moves for zooming, a third lens unit having the negative refractive power, and a fourth lens unit having the positive refractive power, in which the fourth lens unit is movable for focusing.
In the above-mentioned zoom lens of a so-called rear focus type, the focusing mechanism is disposed on an image side of the zooming mechanism. Therefore, compared to a case where an object distance is infinity, in a case where the object distance is on a close-up side of infinity (hereinafter also referred to as finite distance), a zoom ratio (ratio of focal lengths of the entire lens system at a wide angle end and a telephoto end) is reduced. In particular, when the magnification is increased in the above-mentioned zoom lens, the reduction in zoom ratio becomes more conspicuous. In other words, when the magnification is varied while photographing a subject at a small object distance, an image of the subject cannot be magnified to an extent that is expected.
Further, when the increase in magnification is to be achieved in the zoom lens of the above-mentioned type, especially at the telephoto end, an amount of movement of the lens unit for focusing is increased to increase the total length of the zoom lens. This makes the reduction in size of the zoom lens difficult.